Due to increasing amounts of carbon dioxide in the atmosphere resulting from the burning of fossil fuels, global warming is happening. Something needs to be done to at least slow the progression of this increase. There are important parameters to consider when choosing a solution. 
1. The solution must be shown to be relatively environmentally benign.
2. The solution must be shown to be inexpensive. A good target price is $2-5.00 per ton of carbon dioxide.
3. The solution must be permanent. That means the carbon must be sequestered for 1000-10,000 years.
4. The solution must have a large capacity. It must be able to account for all carbon dioxide emitted from fossil fuel burning and in the case that emissions rise, account for them as well. This allows economies to function at optimal levels.
Of all the proposed solutions, only iron fertilization does all of these. Deep sea carbon dioxide injections are both costly and the environmental aspects are uncertain. Reducing carbon emissions is not inexpensive.  Iron Fertilization can do these and maybe return carbon dioxide levels to pre-Industrial times. Iron fertilization is not an expensive process. It will cost about $2.00 per ton of carbon dioxide removed from the atmosphere. Iron Fertilization is very "permanent". Once the phytoplankton bloom occurs, carbon dioxide is more rapidly removed out of the air. As much as 80% of the bloom phytoplankton will sink to the ocean depths.  Since there is so much water in these depth and it moves slowly, the carbon will remain sequestered there for thousands of years. The capacity is not a problem. There is plenty of surface ocean water that can be fertilized.
Methods of fertilization are getting better also. Chemical Engineer, Michael Markels, founder of Ocean Farming, Inc, says that the nutrients and iron are "encapsulated in buoyant, chemically protective containers that keep the nutrients in the "photic zone" longer and release them over time."  Markels is confident that his method works. He suggested that in a 100,000 square mile area, recently purchased by Ocean Farming, Inc., near the Marshall Islands in the Pacific, his method could sequester between a quarter and a third of all the carbon dioxide emitted by the United States. 
The environmentally benign issue is the most debated. Since photosynthesis
is a natural process, that could imply that there is no danger. There
would simply be more photosynthesis going on.
Iron Fertilization meets all of the criteria for a beneficial solution to the carbon dioxide increase in the atmosphere. It is a cheap and long lasting solution. Its environmental danger is at worst unclear. Nothing has been proven harmful.
are other benefits to Iron Fertilization. With the increase in primary
productivity by phytoplankton, this allows their predators (zooplankton
and fish) to increase also. This allows these fertilized waters to
not only reduce carbon dioxide in the atmosphere, but help with food shortages
around the world due to increasing populations of fish. Iron Fertilization,
working with emission reductions, seems to be a good way to regulate the
carbon dioxide in the atmosphere while providing other benefits to society.
 Ocean Farming, Inc. Private
company supporting iron fertilization.
 Scientific American online article.